1. Field of the Invention
The present invention relates to a process for recovering metal values from metal sulphide materials, and particularly from refractory materials, complex copper concentrates and nickel concentrates that contain precious metals.
2. Description of the Related Art
Sulphide material can be processed totally hydrometallurgically, for instance by means of a combination of leaching stages and a subsequent stage in which the leached metal values are recovered with the aid of such methods as liquid extraction, chemical precipitation or electrolysis (electrowinning). Leaching of sulphides can be carried out with oxidizing solvents, such as iron(III)sulphate. A leaching process of this kind, however, is extremely slow and requires considerable space, and is therefore preferably performed outdoors. The sulphides may also be leached subsequent to roasting the sulphides, which converts the valuable metal content to forms which are more readily dissolved and which can be carried out, for instance, in the form of a sulphating or chlorinating roasting process. Sulphide material can also be leached directly with atmospheric oxygen or an oxidation agent, for instance in a sulphuric acid environment under high temperature. A leaching process of this nature must therefore be carried out at elevated pressure in an autoclave. Two reactions occur when pressure leaching in a sulphuric acid environment, these being: EQU MeS+2O.sub.2 .fwdarw.MeSO.sub.4 ( 1) EQU MeS+H.sub.2 SO.sub.4 +1/2O.sub.2 .fwdarw.MeSO.sub.4 +S.sup.0 +H.sub.2 O(2)
of which reactions the reaction (1) is encouraged by high temperatures, It is not normally possible to avoid reaction (2), and consequently the leaching product will contain elemental sulphur (S.sup.0), which is liable to complicate both the leaching process concerned and also continued leaching of the leaching residue, due to inactivation of the surface of the leached material by precipitated sulphur. Pressure leaching has not been used industrially to any appreciable extent, because of the problems indicated above.
Leaching of sulphidic material with atmospheric oxygen can also be carried out in the presence of bacteria as "catalyzing" auxiliaries. In general, these bacteria are comprised of Thiobacillus Ferrooxidans, which encourage the oxidation of both sulphur and iron. Bacteria-based leaching processes are normally carried out with the intention of recovering metals according to one of the following alternatives, both of which are applied on an operational scale in many places in the world.
1. Leaching with the intention of leaching out valuable metals, which are then recovered selectively from the leaching solution with the aid of conventional hydrometallurgical methods. Examples: copper, nickel, cobalt, uranium, zinc.
2. Leaching of so-called refractory minerals that contain precious metals (such as pyrite and arsenopyrite) so as to free the precious metals, which are then extracted from the leaching residue (e.g. cyanide leaching) by means of conventional hydrometallurgical methods.
Bacteria leaching affords certain advantages over pressure leaching, among other things because the bacteria encourage or favour oxidation of both sulphide sulphur and elemental sulphur to sulphate. The oxidation of Fe(II) to FE(III) is also encouraged. The bacteria-leached material can therefore be subjected to further leaching, e.g. with cyanide, to recover precious metals from the first leaching residue, without risk of problems caused by elementary sulphur. One serious drawback, however, is that bacteria leaching requires very long leaching times in order to achieve sufficiently high metal yields. This will be exemplified in the following with reference to the recovery, or winning, of precious metals from so-called refractory materials, although the problem applies generally to the majority of the sulphide materials in question.
Precious metals, particularly gold, are often present in the form of submicroscopic grains embedded in "host minerals" such as pyrite and arsenopyrite. Hydrometallurgical recovery of precious metals from these materials is attractive in most cases, and is preferred to pyrometallurgical processes, which, from a technical and economical aspect, are only conceivable when the material has high precious metal contents and low impurity contents (arsenic, antimony, etc.). It is not possible, however, to recover the precious metal content of such materials directly by conventional extraction with cyanide or thiocarbamide, since the precious metals are not accessible to the chemical reagents. The material, i.e. the ore or the concentrate, must therefore be "loosened" to free the precious metal grains prior to the extraction process. This loosening process is carried out by means of an oxidation process, which can be effected by roasting, pressure-leaching or bacteria-leaching the material concerned, as discussed in the introduction. The precious metals can then be extracted in a conventional manner, by cyanidation.
When the material-loosening oxidation process is effected by leaching, the precious metals are freed while, at the same time, the iron, sulphur and arsenic present sink as a result of oxidation and dissolution. Thus, since these elements take-up a dominating part of the original material, the concentration of the precious metals present in the residue will rise in proportion thereto. One serious problem with the cyanide extraction of precious metals from refractory materials is that the oxidation process must be relatively complete in order to achieve a satisfactory precious metal yield, since remaining non-oxidized sulphide mineral will contain unaccessible precious metals. Total oxidation of sulphide concentrate is both time-consuming and costly, both when effected in an autoclave and when effected with bacteria, which greatly restricts the usefulness of this pre-treatment process, both technically and economically. Thus, the only alternative that remains is roasting, despite all of the obvious environmental disadvantages manifested with such oxidation processes, for instance such drawbacks as the need to effectively clean the flue gases and to treat the dust generated, where requirements can be as high as 99% and even higher with regard to sulphur purification. Quantitatively, the resultant roasted product will have generally the same size as the input material, since the iron remains in the roasted product and oxygen takes the place of the sulphur that is roasted off.